Semiconductor integrated circuits have been increased in integration density, thereby improving functions and performance. Increased integration density of circuitry has been achieved by reducing pattern size, thereby forming the circuit in a smaller domain. That is, pattern size reduction has supported the semiconductor industry.
Conventionally, a photomask is employed during various photolithographic processing phases during the manufacture of the semiconductor device, serving as an original plate for transferring a fine pattern, as to a semiconductor substrate during formation of an integrated circuit. The photomask pattern is transferred at the original size or at a reduced size. The resolving dimension L of a photolithographic pattern is generally expressed as L=k1 X p /NA, wherein: p denotes an exposure wavelength; NA denotes a numerical aperture of a projective optical system; and k1 denotes a constant for a process having a value of approximately 0.5. Therefore, to form a fine pattern, it is necessary to decrease the exposure wavelength p and increase the numerical aperture NA.
In order to form a finer pattern by photolithography, the exposure wavelength has been decreased and a KrF excimer laser beam with a wavelength of 248 nm has been employed as a light source. It is known, however, that the depth of focus (DOF) for pattern transfer is generally expressed as DOF=k2 .times.p /NA.sup.2. Accordingly, the depth of focus DOF decreases as the exposure wavelength decreases. Therefore, it is possible to improve photolithographic resolution by decreasing the exposure wavelength, thereby further improving DOF.
Photolithographic performance can be improved by using the so-called ultra-resolving technique. Typically, however, deformed illumination and phase shift exposure techniques are used. The exposure technique, using the so-called Levenson phase shift mask, is considered the most promising, because the resolution and DOF can be significantly improved. The Levenson phase shift mask is a photomask for modulating phases of exposed rays passing through adjacent opaque-film apertures, creating opposite phases.
Adverting to FIGS. 19(a) and 19(b), conventional practices comprise modulating projected light employing a conventional photomask. FIG. 19(a) illustrates a sectional view of conventional photomask 200 comprising an opaque-film pattern 201 and an opaque-film aperture pattern 202. FIG. 19(b) shows the electric field amplitude after exposure through photomask 200, modulated by the aperture pattern.
FIGS. 20(a) and 20(b) show the electric field of a projected image on a semiconductor wafer on which a pattern of the photomask in FIG. 19(a) is transferred. FIG. 20(a) shows the electric field amplitude on the wafer, with an electric field 204 according to each aperture and a synthesized electric field 205. FIG. 20(b) shows the electric field intensity on the wafer. When a numerical aperture NA of an imaging system becomes less than 1, the electric field components, which change spatially and finely in the modulation information immediately behind the photomask, are removed. Further, an electric field having a moderate fluctuation is formed. The electric field modulated by adjacent apertures also has a moderate fluctuation. As a result, a synthesized electric field with considerable intensity between adjacent apertures is formed, preventing the two adjacent apertures from being separated.
FIGS. 21(a) and 21(b) illustrate how projected light is modulated by a Levenson phase shift mask. FIG. 21(a) illustrates a sectional view of a Levenson phase shift mask 300, including an opaque-film pattern 301, an opaque film aperture pattern 302 without phase shift processing, and an opaque-film aperture pattern 303 with phase shift processing. FIG. 21(b) shows the electric field amplitude immediately after exposure through Levenson phase shift mask 300. As shown in these drawings, modulation of an electric field by an opaque-film aperture is equivalent to modulation by a conventional mask. However, the electric field modulated by adjacent apertures is reversed in phase by the function of the Levenson phase shift mask.
FIGS. 22(a) and 22(b) illustrate the electric field of a projected image of a Levenson phase shift mask on a semiconductor wafer, on which a transferred pattern is formed by the Levenson phase shift mask shown in FIG. 21(a). FIG. 22(a) shows the electric field amplitude on the wafer, with an electric field 304 and a synthetic electric field 305. The image of each opaque-film aperture formed by an optical system with numerical number NA has a moderate fluctuation which is similar to a conventional mask. However, unlike the conventional mask, phases of adjacent apertures are reversed, offsetting the electric fields between the apertures. Therefore, the images of the adjacent two apertures are separately formed. Thus, high-resolution imaging results.
As shown above, the exposure resolution is improved using a Levenson phase shift mask. However, a normal Levenson phase shift mask has the following problem. When a dark pattern including a fine dark line is formed, the dark pattern is always angularly formed, and cannot be formed as an open line. In other words, the dark pattern can only be formed as a multiply-connected domain, including one or more simply-connected bright domain or domains. This occurs because the Levenson phase shift mask is formed to provide either a 0 p or a 180 p phase shift.
FIGS. 23 to 26 illustrate a conventional Levenson phase shift mask having two kinds of phases with a phase shifter formed by etching a quartz substrate. FIGS. 27 to 29 illustrate the formation of a fine dark line pattern by the above-mentioned Levenson phase shift mask.
FIG. 23 is a perspective view of a conventional phase shift mask 400, and FIG. 24 is a top view of the phase shift mask 400. FIG. 25 is a sectional view of the phase shift mask taken along line A--A in FIG. 24. FIG. 26 is a sectional view of the phase shift mask taken along the line B--B in FIG. 24.
As shown in FIGS. 23 to 26, in forming a fine dark line pattern, the opaque-film pattern 405 on quartz substrate 401 is sandwiched by opaque-film aperture patterns 402 and 403. In this case, unless the dark line connects two points on a boundary specifying the whole domain for a mask formation (i.e., unless the dark line is connecting opposite ends of a mask), at least one of the two opaque-film aperture patterns 402 and 403 is closed. In FIGS. 23 to 26, opaque-film aperture portion 403, with shifter processing, is formed as a closed pattern. Opaque-film aperture portion 402, without shifter processing, is formed to surround opaque-film aperture portion 403. Opaque-film pattern 405, corresponding to a required dark line, is formed between opaque-film aperture patterns 402 and 403.
FIGS. 27(a) and 27(b), and FIGS. 28(a) and 28(b), illustrate transmission of light applied vertically to the phase shift mask. FIG. 27(a) shows a cross section of opaque-film 405 for forming a dark line of the phase shift mask. The intensity of light passing through the cross section causes a dark line portion due to opaque-film 405, as shown in FIG. 27(b). FIG. 28(a) shows the cross section of a boundary portion of the phase shift domain of the phase shift mask. The intensity of the light passing through the cross section causes a dark line portion as shown in FIG. 28(b).
FIG. 29 shows a resist pattern formed on the positive resist on a wafer by the phase shift mask 400. A dark line image is formed on a portion corresponding to the opaque-film pattern 405, and resist pattern 406 is formed, since the positive resist is not dissolved due to development. Resist pattern 406 is formed as required, however, a dark line image is also formed at the boundary portion between opaque-film aperture patterns 402 and 403. The opaque-film pattern 405 is not present. This results because phases of the transmitted rays are reversed from each other on both sides of the boundary. Thus, a resist pattern 407 is formed on the wafer. That is, an open linear pattern is not formed but, a closed pattern including unnecessary pattern 407 is formed.
Therefore, to form a line pattern including an open line, a negative resist whose resolution is generally inferior to that of a positive resist must be used. Accordingly, adequate resolution cannot be obtained.
A method for addressing the problem in the conventional technique has been proposed in which the phase difference of the transmitted light in opaque-film apertures on both sides of a linear opaque-film pattern is 180 p. This is accomplished by changing the phase of transmitted light continuously between the apertures and, as a result, the entire domain surrounding the linear opaque-film pattern becomes a bright domain. An example of such a method is described in "Proc. SPIE 2440, p. 515", wherein portions having phase difference of 5gradations, i.e., 0 p, 45 p, 90, 135 p, and 180 p, are stepwise formed in one opaque-film aperture in order to continuously change the phase of transmitted light. Thus, a dark line image is prevented from occurring due to adjacent 0 p and 180 p portions. In this example, however, resist patterning and etching are performed four times in order to form a practically continuous phase transition portion. In this method, in order to manufacture a mask having n+1 phase gradations, resist patterning and etching must be performed n times respectively. Therefore, since many more patterning and etching steps are required, the production cost is increased, and the yield lowered.